Forbid consecutive slices

Rule request

Thesis

We should forbid consecutive slices.

Reasoning

Consider this code:

>>> a = [1, 2, 3, 4]
>>> a[1:][:2]
[2, 3]

What it does?

1. a[1:] is [2, 3, 4]
2. [2, 3, 4][:2] is [2, 3]

It is much more readable and effective to use

>>> a[1:3]
[2, 3]

Related https://twitter.com/m_ou_se/status/1403426221367627777

[Edald123]

Hi! I actually worked on a solution with Gibran.

So far we have implemented a solution in wemake_python_styleguide/visitors/ast/subscripts.py. In the class SubscriptVisitor we added:

def _check_consecutive(self, node: ast.Subscript) -> None:
        """Check if subscript node has a slice and a subscript"""
        if isinstance(node.slice, ast.Slice) and isinstance(node.value, ast.Subscript):
            self.add_violation(consistency.ForbidConsecutiveSlicesViolation(node))

Our implementation is passing some tests but others not. The tests we created are for:

no_consecutive = """
a = [1, 2, 3, 4]
b = a[1:]
"""

no_consecutive_visit = """
a = [1, 2, 3, 4]
b = a[1:][2]
"""

no_slicing = """
a = [1, 2, 3, 4]
b = a[1]
"""

no_slicing_double = """
a = [1, [5, 6, 7], 3, 4]
b = a[1][3]
"""

no_slicing_triple = """
a = [1, [5, 6, 7, [8, 9, 10]], 3, 4]
b = a[1][3][1]
"""

consecutive_double = """
a = [1, 2, 3, 4]
b = a[1:][2:]
"""

consecutive_triple = """
a = [1, 2, 3, 4, 5, 6]
b = a[1:][2:][:2]
"""

consecutive_plus = """
a = [1, [5, 6, 7, 8, 9, 10], 3, 4]
b = a[1][2:][:4]
"""

no_consecutive_for_slices = """
a = [1, [5, 6, 7, [8, [1, 2, 3, 4], 10]], 3, 4]
for i in a[1][3][1]:
    print(i)
"""

consecutive_for = """
a = [1, [5, 6, 7, [8, [1, 2, 3, 4], 10]], 3, 4]
for i in a[1][:4][1:]:
    print(i)
"""

The tests that are not supposed to raise errors are: no_consecutive, no_consecutive_visit, no_slicing, no_slicing_double, no_slicing_triple, no_consecutive_for_slices. These are working fine.

The tests that are supposed to raise an error (violation) are: consecutive_double, consecutive_triple, consecutive_plus, consecutive_for. Of these tests only consecutive_double is passing since we are expecting the raise of 1 violation and it is raising 1 The other three are failing: consecutive_triple, consecutive_for, and consecutive_plus.

The problem is that these 3 tests are raising 2 errors instead of 1 so they fail: AssertionError: assert 1 == 2 This failure is occurring in tests/test_visitors/conftest.py, in the assert_errors function, in the line assert len(errors) == len(real_errors) Therefore, it seems like the code is working for forbidding 2 consecutive slices, but when there are more it isn’t. We are working on solving this.

[sobolevn]

Oups, sorry! I’ve totally missed it (because it was not linked to this issue). This PR is merged: https://github.com/wemake-services/wemake-python-styleguide/pull/2220